3, x³-y³=(x-y)(x²+xy+y²)
5a, (125a³b^4c^5+10a³b²c²)÷(-5a³b²c²)
=-25b²c³-2
b, (8x²-26x+21)÷(2x-3)=4x²-6x-22 dư 45
6,a, 3cx²(x-1)-6x(1-x)=3cx²(x-1)+6x(x-1)
=(3cx²+6x)(x-1)=3x(cx+2)(x-1)
c, 5x(x-1)=x-1
⇒5x(x-1)+x-1=0
⇒(5x+1)(x-1)=0
⇒5x+1=0⇒x=1/5
⇒ x-1=0⇒x=1
d, (a+b)²-(a-b)²=(a+b-a+b)(a+b+a-b)=2b·2a=4ab